Hydrophobic gas filters are used for inlet gas and vent filters on fermentors and bioreactors, as well as other process equipment. Hydrophobic filters will not wet in water but will wet in low surface tension liquids, for example, organic solvents such as alcohols. Once a hydrophobic filter has been wetted, aqueous solutions also will pass through. Hydrophobic filters are best suited for gas filtration and venting arrangements. In certain applications, hydrophobic filters are used to filter aqueous solutions because of compatibility requirements. Water or aqueous solutions can also pass through a hydrophobic filter once the water breakthrough pressure is reached.
When used with a fermentor, a vent filter is subjected to high volume and/or continuous flow rates of gases that have passed through a culture medium. As a result, these gases typically have high humidity levels and often entrain droplets of the culture media. When this humid gas stream passes through a hydrophobic filter, it deposits some of the entrained fluid on (or within) the filter material and hence begins “wetting” the filter material. Since the material is hydrophobic, as the fluid wets the filter, its ability to pass the gas stream diminishes, until eventually the filter becomes blocked with fluid and will no longer allow the gas to pass through. This decreasing flow capacity causes problems with the pressure control, as well as gas inlet flow, on the fermentor until it reaches a point where pressure control is no longer possible and the current fermentation batch is lost. This has been a problem in the biotechnology industry for years and also applies to bioreactors and the filters associated with their processes. Many other processes, such as in the medical field, utilize hydrophobic filters and need to constantly monitor and/or change the filter elements to overcome the wetting problem.
Attempts to solve this problem in the prior art have used pre-filter vent gas heaters, pre-filter vent gas condensers, steam jacketed filter housings and external electric heaters for the filter housing. The condensers attempt to remove the moisture, while the pre-heaters attempt to vaporize the moisture, prior to entering the filter. Both of these options are relatively expensive and must be “sized” for each application on a case-by-case basis. Indeed, the condensers and pre-heaters often become large and expensive for high glass flow rate applications. Steam jacketed filter housings and external electric heaters suffer from the large physical gap between the filter element and the housing, which typically has a high flow rate gas passing through it, providing a large thermal barrier to heat transfer from the external heater to the filter element.
Thus a need remains in the art for a system that provides the desired drying to the filters without the size limitations and marginal effectiveness of the prior art.